Ageing occurring during storage in printed and paper products and in particular in books leads to progressive damage to the paper substance, particularly due to traces of acids released in the paper, but also due to other processes.
Depending on the degree of this damage, the papers become friable or possibly decompose completely after a few decades.
To prevent the progress of acid damage, processes are known in which the papers are subjected to neutralization treatment as single sheets or in bound form.
Continuing damage to the papers is indeed prevented using this treatment; however, adequate strengthening of papers already damaged is not achieved without measures additional to the neutralization treatment.
To maintain particularly valuable archives, strengthening processes are known in which the paper is treated in the form of individual sheets. For books the book block must be opened for this purpose.
Processes of this type consist in alkaline treatment, in which the individual sheets are passed through an alkaline bath, in chamfering, in which a cellulose fibre layer is suspended on the paper, and in gluing-on strengthening films, for example of Japanese paper. In a particularly complex variant of this process, the sheet of paper is split and the strengthening layer is sized between the sheet halves. These processes are described in detail by H. Bansa in the article "Neue Entwicklungen auf dem Gebiet der Konservierung" [New developments in the field of preservation], Zeitschrift fur Bibliothekswesen und Bibliographie, Year 35, Part 3, pages 226 to 236 (1988).
In view of the very large number of books--a couple of hundred million books are to be treated worldwide--these complex processes are disadvantageous for mass treatment.
Only processes which operate using subsequent sizing of the paper and in which it is possible to treat whole books or paper pages combined in another fashion, are suitable for this.
European application 0 273 902 indicates a process for this, in which the books are treated with an aqueous calcium hydroxide solution also containing 0.1-1% of dissolved methyl cellulose. Traces of free acid are neutralized by the calcium hydroxide, while the filmforming methyl cellulose strengthens the paper after rapid freezing and freeze drying the books.
The freeze drying used, which may be up to 34 hours, is an obstacle to rational treatment of large numbers of books.
Solutions of polymeric materials, for example methyl cellulose, may also lead to increasing the viscosity of treatment solutions, which makes it difficult for uniform and reliable penetration of entire book blocks. Also in this case care must be taken to prevent bonding of individual pages.
Processes being developed, which provide treatment of the books using non-aqueous solutions of monomeric but polymerisable materials, for example methyl methacrylate, ethyl acrylate or dimethyl aminomethacrylate, take into account this problem of viscosity increase. The polymerization of the monomers is effected by the action of ionizing rays.
By incorporating a polymer into the fibrous structure of the paper, strengthening is indeed possible, but this process requires the use of complex techniques.
An object of the present invention is to provide an improved restrengthening process and product, in which the disadvantages mentioned are avoided and which is particularly suitable for efficient treatment.